The present invention relates to an overdrive device for use in an automatic transmission in a motor vehicle.
There has arisen a demand for an automobile transmission which may establish a reduction gear ratio of less than 1 (overdrive). Several purposes may be served by such a transmission. They may include: reducing the amount of harmful constituents of exhaust gases from the engine; improving the high speed running performance of a vehicle on a road such as a freeway; and reducing noise during the operation of the engine.
An overdrive gear position in an automatic transmission, in which the torque from the engine crank shaft is automatically converted commensurate with the running condition of the motor vehicle for transmission of the torque to the drive wheels, may be achieved by means of one or more auxiliary transmissions establishing two or more reduction gear ratios, respectively, in addition to an overdrive device providing a reduction gear ratio of less than 1.
In general, it has been a common practice in design of automatic transmissions with an overdrive device, that a minimum reduction gear ratio be used most frequently, with the result that the gear ratio corresponding to an overdrive is used most frequently. The overdrive device includes planet pinions, a sun gear in mesh with the planet pinions, and a ring gear coupled to an output shaft and meshing with the planet pinions. Thus, upon establishing the overdrive gear position, the sun gear is fixed, while the planet pinions revolve around the sun gear, turning on their axes, thereby rotating the ring gear. In other words, unlike the solid coupling of an input shaft to an output shaft, there results differences in r.p.m. among respective components of the overdrive device.
For instance, assuming a number of teeth Zs of a sun gear, a number of teeth Zp of a planet pinion, and an r.p.m. C of a carrier, then the difference in r.p.m. is given as Zs/Zp x C. A needle bearing is disposed between each planet pinion and pinion shaft in a carrier so as to rotatably support the planet pinion thereon. In this respect, moments in opposing directions are produced by the sun gear and ring gear due to an inclination of the teeth of each gear, so that a normal force in the axial direction acts outwardly on each needle in a needle bearing.
On the other hand, tangential forces in the direction of a rotating carrier are applied between the sun gear and the planet pinions and between the planet pinions and the ring gear, so that a force in the direction of the rotating carrier acts on each needle in the bearing. As a result, a resultant force derived from a normal force and a tangential force acting on each needle is concentrated on the opposite ends of each needle. This leads to skew and deformation in the needle bearing, thereby shortening the service life thereof.
In addition, the skew and deformation thus caused in the needle bearing in turn cause planet pinions to produce a thrust, thereby accelerating wear of thrust washers positioned between a carrier and planet pinions.
Furthermore, a large difference in r.p.m. among the aforementioned gears then produces a considerable quantity of heat in the needle bearings, while the skew and deformation of the needle bearings leads to an additional increase in the quantity of heat thus produced. Heat is produced at meshing points of respective gears in an overdrive gear position, as well. The heat caused due to the foregoing various reasons lowers the durability of elements incorporated in the needle bearings, and results in lowered performance thereof.
Accordingly, the present invention is directed toward provision of an overdrive device for use in an automatic transmission which improves the durability of the needle bearings.
Furthermore, the invention is intended to provide an overdrive device of the type which may reduce the size of an automatic transmission.